Magnetically Responsive Colloidal Nanoparticles Prepared With Miniemulsion Process And Applied In Consitruciton Of Photonic Crystal Films

Photonic crystals are optical materials consisted of the alternate ordered arrangement of different refractive index medium.It can form a photonic band gap and reflect electromagnetic waves of specific wavelengths.When the wavelengths locate in the visible region,the photonic crystal shows bright structural color.According to Bragg’s equation,the photonic band gap can be controlled artificially by changing the spacing of diffraction levels,the effective refractive index of the medium and the incident angle.Magnetic assembly of photonic crystals are based on ordered arrangement of monodisperse magnetic responsive colloidal nanoparticles under a magnetic field,and the photonic band gap can be tuned by the magnetic field intensity.Meanwhile,magnetic assembly has the characteristics of instantaneity and reversibility,so it is a fast method to prepare photonic crystals.Magnetic colloidal nanoparticles are the building blocks of magnetically assembled photonic crystals,which are generally composed of magnetic cores and non-magnetic coatings.In the dissertation,the magnetic colloidal nanoparticles were prepared in one step through miniemulsification and self-assembly of an amphiphilic random copolymer poly methyl methacrylate-co-methacrylic acid（P（MMA-co-MAA））to obtain Fe₃O₄@P（MMA-co-MAA）,and P（MMA-co-MAA）was used as an emulsifier and coating material.The structure color could cover the whole visible light band under the control of magnetic field.Fe₃O₄@Si O₂ magnetic colloidal nanoparticles with adjustable size and controllable magnetism were further prepared,and the magnetic core and shell of particles were precisely regulated by changing the conditions of miniemulsion and St?ber system.It could be utilized for designing photonic crystal films with a variety of functions.The main research contents and results are as follows:Fe₃O₄@P（MMA-co-MAA）magnetic colloidal nanoparticles were prepared in one step by the self-assembly of the amphiphilic copolymer using P（MMA-co-MAA）both as an emulsifier and coating material,and the miniemulsion droplets as the nanocontainers.The morphology,magnetic material content and saturation magnetism of Fe₃O₄@P（MMA-co-MAA）NPs were investigated by TEM,SEM,TG,VSM,Zeta potential and nanometer particle size analyzer,FT-IR and fiber optic spectrometer.The effects of the monomer ratio of MMA to MAA,P（MMA-co-MAA）concentration,the amount of oleic acid modified Fe₃O₄（Fe₃O₄-OA）and spin evaporation temperature on the particle size,polydispersity and structure color of Fe₃O₄@P（MMA-co-MAA）were investigated.The optimum experimental conditions for the preparation of Fe₃O₄@P（MMA-co-MAA）by miniemulsion self-assembly were determined as the molar ratio of MMA and MAA at 8:2,0.5%（W/V）P（MMA-co-MAA）,the oil/water ratio of 10:1,100 mg Fe₃O₄-OA,and evaporation temperature at 40℃.The particles prepared under these conditions had an average diameter of 105.0 nm（RSD=6.9%,n=100）.The magnetic substance content was 84.48%`and the saturation magnetism was 69.96 emu·g^-1.The reflection wavelengths of photonic crystal could be tuned from 741 nm to 356 nm,covering the whole visible spectrum as the magnetic field intensity changed in a range of 20 m T-120 m T.Polymer concentration was a key factor affecting particle sizes,monodispersion,and the optical properties of photonic crystals.As the polymer concentration increased,the particle sizes decreased and the surface charge density increased,leading to the red shift of the structural color of the photonic crystal.This method could prepare magnetic colloidal nanoparticles within 30 min and provide a new strategy for the preparation of photonic crystals.A simple and rapid method of preparing Fe₃O₄ magnetic nanoclusters（Fe₃O₄ MNCs）with adjustable particle size was proposed based on miniemulsion process.The Fe₃O₄@Si O₂magnetic colloidal nanoparticles with adjustable shell thickness and designed magnetic strength were obtained.The Fe₃O₄@Si O₂ nanoparticles and optical properties of photonic crystals were characterized by TEM,VSM,Zeta potential and particle size analyzer,and micro fiber optic spectrometer.The effects of the surfactant concentration of lauryl sodium sulfate（SDS）,oil-water phase ratio,the amounts of Fe₃O₄-OA and TEOS on the particle sizes and polydispersity of Fe₃O₄ MNCs and Fe₃O₄@Si O₂,as well as the saturation magnetism of Fe₃O₄@Si O₂ and photonic crystal structural color were investigated.The amounts of Fe₃O₄-OA and TEOS were key factors affecting the particle sizes and saturation magnetism.As the amount of Fe₃O₄-OA increased from 0.05 g to 0.45 g,the particle size increased from 59.0 nm to 99.1 nm.When the amount of TEOS increased,the thickness of the Fe₃O₄@Si O₂ particle shell increased.As the particle size increased from 128.5 nm to 143.7 nm,the saturation magnetism of Fe₃O₄@Si O₂decreased from 47.0 emu·g^-1 to 35.2 emu·g^-1.In the range of 25 m T-200 m T,the controllable range of structural color became narrow,and the whole band had a redshift.This method reduces the preparation time of magnetic colloidal nanoparticles,and the Fe₃O₄ MNCs can be synthesized within 30 min.The Fe₃O₄ MNCs and shell of the particles can be precisely regulated by changing the amount of Fe₃O₄-OA and TEOS.Fe₃O₄@Si O₂ particles can assemble into photonic crystals in methanol,ethanol,DMF,DMSO and other organic solvents,and the structural color had a red shift as the increase of refractive coefficient of the medium.By designing the Fe₃O₄ MNCs sizes and shell thickness of Fe₃O₄@Si O₂ particles,it could be widely applied in photonic crystal polymer films.Fe₃O₄@Si O₂ nanoparticles prepared by miniemulsion method were dispersed in the solution containing the monomer acrylamide（AM）and the humectant glycerol,and the humidity responsive Fe₃O₄@Si O₂/PAM/Glycerol photonic crystal films were prepared through magnetically assembly in an external magnetic field and photopolymerization under ultraviolet light simultaneously.The effects of amount of TEOS,concentration of crosslinking agent and glycerol content on the structure color of photonic crystal response to different relative humidity were investigated by the characterization of reflection spectra.The optimum experimental conditions were determined as follows:TEOS amount was 0.9 m L,crosslinking agent concentration was 2 mg/m L,glycerol content was 75 wt%.When the relative humidity changed in the range of 11%-93%,the reflection wavelengths of the film redshifted from 436 nm to 652nm.The wavelength redshift reached 216 nm,which could respond to the relative humidity in the range of full visible light.When the photonic crystal film in an environment with high relatively humidity,the hydroxyl groups of glycerol in the film could adsorb water from the air through hydrogen bonding,which caused the swelling of polyacrylamide hydrogel.The spacing of particles became wider as the volume of hydrogel increased,which resulted in the increase of lattice spacing of photonic crystal and the redshift of structural color.After the film was placed for 60 days and cycled for 15 runs,the reflected wavelength of the photonic crystal was almost the same,indicating that the film had good stability and repeatability.Through the establishment of standard colorimetric card,a kind of film sensor to express humidity changes was obtained.The humidity sensor realized the long-term visual monitoring of the environmental humidity.The Fe₃O₄@Si O₂ magnetic colloidal particles were embedded in agarose hydrogel to prepare photonic crystal anti-counterfeiting film with a magnetic switch to control the show/hide photonic crystal patterns.The fixation of photonic crystal structure by agarose hydrogel was studied by reflection spectra characterization.The effect of the concentrations of agarose and Fe₃O₄@Si O₂ magnetic colloidal particles on the show/hide effect of photonic crystal patterns were investigated.The optimal experimental scheme was determined as 1 wt%of agarose concentration and 3 mg/m L of magnetic particle concentration.Binding force of agarose hydrogels on particles was key factor during the printing and show/hide process of photonic crystal anti-counterfeiting pattern.Agarose aqueous solution containing Fe₃O₄@Si O₂magnetic colloidal particles was rapidly cooled into gel under a pattern composed of the magnets,and the photonic crystal anti-counterfeiting pattern is formed instantaneously under the magnetic field.The photonic crystal hydrogel formed based on the balance between the solvation layer repulsion of Si O₂ shell and magnetic attraction of Fe₃O₄@Si O₂particles.Once the magnet was removed,the magnetic attraction disappeared and the balance disrupted.The repulsive force drove the particles move away from each other and resulted in the instant disappearance of the anti-counterfeiting pattern.When the magnetic field was applied again,the nanoparticles at the position of the anti-counterfeiting pattern reassembled into photonic crystal structure,while the particles outside the pattern could not form an ordered structure due to the agarose hydrogel obstruction.Thus,the anti-counterfeiting pattern as well as the effect of magnetic switch show/hide were realized.The anti-counterfeiting film had fast preparation speed and simple process,and the printing of anti-counterfeiting pattern and the preparation of the film could be finished within 1 min.It will be applied in the field of anti-counterfeiting labels.A photonic crystal film shielding both ultraviolet and blue light was prepared by embedding the magnetic assembly photonic crystal and the UV absorber Ti O₂ nanoparticles into the hydroxyethyl polyacrylate（PHEA）film.The shell thickness of Fe₃O₄@Si O₂ particles decreased by reducing the amount of TEOS,and the saturation magnetization intensity increased correspondingly.Therefore,the reflection wavelength of the magnetically assembly colloidal photonic crystal can be adjusted to cover the blue light region（428 nm-556 nm）to block the blue light.Ti O₂ nanoparticles were modified by triethanolamine（TEA）to improve its dispersibility in monomer HEA solvent.The influences of the concentrations of Ti O₂-TEA and Fe₃O₄@Si O₂ particles on the shielding effect of the film were investigated by measuring the UV absorbance of the films.The optimal reaction conditions of TEA modified Ti O₂ were determined as 6 m L TEA,vortex time of 10min,ultrasonic treatment time of 60 min and stirring rate at 500 rpm.When the content of Ti O₂-TEA in the film increased from 5 wt%to 20 wt%,the transmittance at 360 nm decreased from 75.2%to 15.8%.The shielding for UV light and blue light was derived from Ti O₂ particle and photonic crystals structure,respectively.When the concentrations of Ti O₂ and Fe₃O₄@Si O₂ were 20 wt%and 1 mg/m L,the shielding rates of the film at 360 nm and 450 nm was 84.2%and 42.8%.The film had good tensile properties and was insensitive to water,and it was expected to be specialty window film products applied in automobile,architecture and other areas.